1. Field of the Invention
The present invention relates to a process for preparing bismuth compounds useful for use in electronic devices such as ferroelectric memories.
2. Description of the Related Art
Bismuth compounds, in particular bismuth layer compounds, are a class of compounds which are industrially very important in bismuth superconductive oxides having a critical temperature of 110K, materials for ferroelectric memories or the like. In order to apply these compounds to electronic devices, a process for forming thin films must be developed.
For these bismuth layer compounds, for example, Bi.sub.2 (Sr,Ba,Ca)(Ta,Nb).sub.2 O.sub.9, a temperature for heat treatment in a process for preparing a bulk is 1,000.degree. C. or higher (see G. A. SMOLENSKII et al., SOVIET PHYSICS-SOLID STATE, p.651-655 (1961)) while a temperature for post annealing in a process for preparing a thin film is about 800.degree. C. (see C. A. Paz de Araujo et al., International Patent Publication #W093/12542 (Jun. 24, 1993)).
The above mentioned high heat treatment temperature may cause various problems with respect to semiconductor processes.
In capacitors using a bismuth layer compound, for instance, hillock (i.e., growth of coarse particles) may be caused in a platinum lower electrode which becomes one of electrodes of a capacitor, or a buffer layer which is formed between the platinum lower electrode and an electrode-forming surface thereof may deteriorate in the heat treatment due to a problem of its heat resistance.
When titanium nitride TiN is used as a buffer layer, for instance, the annealing temperature should be decreased to 700.degree. C. at the highest, preferably to about 650.degree. C. or lower.
Illustrative methods for decreasing the annealing temperature include development of a new starting material or film-formation as deposited, i.e., in the state of a film as simply formed by a ultra-high vacuum process such as MOMBE (metal organic molecular beam epitaxy; CBE). At present, however, these methods could not have crystallized oxides such as bismuth layer compounds.